This disclosure relates to releasable connections between cylindrical members or bodies. In some aspects, this disclosure relates to connections between downhole tubulars, such as drill pipe tool joints, as are employed in the rotary system of drilling. More particularly, the downhole tubular connections or drill pipe tool joints include connections configured to be selectively disconnectable within the well bore, such that upper and lower portions of the downhole tubular string can be separated.
In drilling by the rotary method, a drill bit is attached to the lower end of a drill stem composed of lengths of tubular drill pipe and other components joined together by tool joints with rotary shouldered threaded connections. In this disclosure, “drill stem” is intended to include other forms of downhole tubular strings such as drill strings and work strings. A rotary shouldered threaded connection may also be referred to as a RSTC. Furthermore, the tubular members that make up a drill stem may also be substituted with other rods, shafts, or other cylindrical members that may be used at the surface and which may require a releasable connection.
The drill stem may include threads that are engaged by right hand and/or left hand rotation. The threaded connections must sustain the weight of the drill stem, withstand the strain of repeated make-up and break-out, resist fatigue, resist additional make-up during drilling, provide a leak proof seal, and not loosen during normal operations.
The rotary drilling process subjects the drill stem to tremendous dynamic tensile stresses, dynamic bending stresses and dynamic rotational stresses that can result in premature drill stem failure due to fatigue. The accepted design of drill stem connections is to incorporate coarse tapered threads and metal to metal sealing shoulders. Proper design is a balance of strength between the internal and external threaded connection. Some of the variables include outside diameter, inside diameters, thread pitch, thread form, sealing shoulder area, metal selection, grease friction factor and assembly torque. Those skilled in the art are aware of the interrelationships of these variables and the severity of the stresses placed on a drill stem.
The tool joints or pipe connections in the drill stem must have appropriate shoulder area, thread pitch, shear area and friction to transmit the required drilling torque. In use, all threads in the drill string must be assembled with a torque exceeding the required drilling torque as a minimum, or more to handle tensile and bending loads without shoulder separation because shoulder separation causes leaks and fretting wear.
Drill stem and tool joints with rotary shouldered threaded connections are addressed by industry accepted standards such as, but not limited to: International Industry Standard (ISO), ISO 10424-1:2004 (modified)—Part 1 and Part 2; Petroleum and natural gas industries-Rotary drilling equipment—Part 1: Rotary drill stem elements; American Petroleum Institute (API), API 7-1 Specification for Rotary Drill Stem Elements; API 7G Recommended Practice for Drill Stem Design and Operating Limits; and others. These standards address design, manufacture, use and maintenance of drill stem thread joints.
Offshore drilling, for example, is performed in progressively deeper water, with deeper penetration of the earth and possibly having higher deviations from a vertical bore hole. Further, many wells now have sections of horizontal bore hole. The temperatures are high, the friction between drill stem and borehole is high, the hanging weight is extreme and the well bores may not be straight. Consequently, a portion of the drill stem often becomes stuck at a great distance from the surface, preventing its movement and recovery. Further causes of stuck drill pipe include accumulation of cuttings falling out of circulated fluids, unconsolidated earth caving in the borehole, low pressure strata capturing the drill stem due to differential pressure, and the like.
In rotary drilling, to remove stuck drill stem from the well, typically the first remedial action is to identify the point at which the drill stem is stuck. A decision is then often made to either explosively loosen a thread joint or sever the drill stem with highly reactive explosive or chemical tools.
Highly specialized crews, equipment and tools must be mobilized and transported to the well location. The transportation of explosive or highly reactive chemical tools is subject to tight governmental regulation. The use of such explosive or highly reactive chemical tools presents a risk to the well operation in that the possibility exists that accidental discharge on the surface can cause property damage and injury or death to personnel. Mobilization and transportation can consume a significant amount of expensive, non-productive time.
Current tubing disconnects designed for within-the-well-bore activation are lacking. In the discussion below, tubing disconnects are disposed in work strings or pipe strings used for coiled tubing drilling, well completion, workover, and other services less demanding than rotary drilling. Consequently, current tubing disconnects do not meet the requirements of the ISO and/or API specifications for rotary drilling equipment. They incorporate non-shouldered connections and/or connections that do not establish a stress pattern within the connection to prevent shoulders from separation under extreme tension and/or bending loads.
Further, current tubing disconnects employ non-metal seals. Failure of one of these seals may result in a washout or total joint failure. The sliding fit of the seals facilitates fretting and wear as the tubing is flexed in response to axial, bending and rotational forces. Some tubing disconnects make use of springs or washers that restrict the inside diameter of the tubing string, or leave a connection in the well that cannot be easily reconnected without special tools. Some tubing disconnects leave a ball or other activation device in the well that can inhibit additional work that may be required after recovery of the upper unstuck tubing section.
Various current tubing disconnects are intended for very specific applications. Often, the environment in which these tubing disconnects are operated is relatively stable and predictable. For example, such tubing disconnects are intended for releasing perforating guns after firing, sub-sea risers, or coiled tubing drilling bits. However, such tubing disconnects do not have the ruggedness and are not designed to operate in the extremes of rotary drilling. Such tubing disconnects often include mechanical features that a driller would recognize as a weak link in a rotary drill stem.
Some current tubing disconnects include pressure activation, requiring the ability to circulate fluids within the well tubing. Pressure activated tubing disconnects are typically activated by dropping or pumping a ball or like device to engage a seat, so that pressure may be applied down the tubing to initiate disconnection. Without circulation, differential pressure cannot be reliably established to disconnect the tubing. The seat is a restriction to and subject to damage by the passage of instruments such as measurement while drilling tools and the like. Accidental impact of tools passing the seat may initiate inadvertent and unwanted disconnection. If the well tubing is plugged, a circulation port must be opened before disconnection is possible. A circulation port degrades the reliability and pressure integrity of the tubing.
In the process of drilling a well, a drill bit and drill stem may drill a significant distance into the earth without requiring removal and refitting of a new drill bit. It is problematic to determine where to install a disconnect within the drill stem. Deciding the optimum location of a disconnect requires an accurate estimation of the probable depth of the portion of the drill stem that has become stuck. This problem is compounded because a disconnect is lowered progressively deeper as the well is drilled. The tubing or drill stem may become stuck due to solids, such as sand, falling out of well fluid suspension at any depth within a well.
The several embodiments described herein overcome these and other limitations in the art. By way of example, and in no way limiting the scope of this disclosure, a downhole tubular string disconnect mechanism in accordance with the principles disclosed herein may be configured for selective activation within the well bore, meet industry standards and/or expectations of ruggedness for rotary drilling and other downhole applications, be insensitive to the passage of instruments and tools, not require well fluid circulation, not require pressure application to the drill stem, not leave an obstructed well bore after disconnection, and allow disconnection of a drill stem at selectable, multiple locations by installing multiple disconnects along the length of the drill stem and providing the ability to disconnect the lowest one in the unstuck portion of the drill stem. Other limitations are also overcome, including for cylindrical member couplings such as for drive shafts.